In a read-only optical recording medium, such as a compact disc (CD) read only memory (ROM) or digital versatile disc (DVD) ROM, user data is built into the disc and also provides position and timing information for reading the data. However, in a recordable CD or DVD, such position and timing information must be provided without user data. Thus, recordable CDs or DVDs are manufactured with preformatted land-groove patterns in order to encode position and timing information. Fine position information comes from the “push-pull” signal arising from the presence of the alternating land and groove patterns. Timing information, which is used to write the user data at a precise circumferential location of the groove, is provided by “wobbling” the grooves. Coarse position information is encoded in the wobble, or on the land adjacent to the groove, depending on the format. The optical disc driving system recovers the position and timing information from the preformatted patterns using a timing loop. The exact method of information recovery depends on the precise format of the recordable CD or DVD.
There are various types and formats of recordable optical discs, for example, CD-R (recordable), CD-RW (rewriteable), DVD-R (recordable), DVD+R, DVD-RW, DVD+RW, and the like. DVD-R and DVD-RW use preformatted pits on the land adjacent to the track to provide the position information, and the groove is wobbled at fixed frequency to provide the timing information. The timing information is used to generate a write clock. On the other hand, DVD+R and DVD+RW encode the position data using a single cycle Binary Phase Shift Keying (BPSK) modulation on the wobble, so that a small set of the wobble cycles are opposite in phase to the rest. The remaining cycles form a constant frequency sinusoid similarly to DVD-R/RW, and provide timing information for the write clock. On the other hand, CD-R and CD-RW use frequency modulation (FM) to encode a bit stream containing sync information and data block number. Consequently, the wobble signal from a recordable CD takes two values centered on the desired wobble frequency (the carrier frequency).
In order to recover these preformatted timing and position information (also referred to as wobble data), a timing loop is conventionally used in CD-recordable and DVD-recordable chip sets. FIG. 1 schematically illustrates a conventional timing loop 10. The timing loop 10 typically includes a phase detector 12, a loop filter 14, and a Voltage Controlled Oscillator (VCO) 16. The timing loop 10 is commonly referred to as a phase lock loop (PLL). The recording system for either format is required to have an accurate timing loop to demodulate the encoded coarse position information and properly align the write data with the wobble. A DVD-recordable system only requires one timing loop because, in a DVD-recordable format, the timing information is provided by the wobble with a fixed frequency and thus the wobble data is synchronous with the timing wobble signal. However, in a CD-recordable format which uses frequency modulation of the carrier signal, the wobble data is modulated onto the wobble signal somewhat asynchronously. Thus, a CD-recordable system requires two different timing loops: one for the wobble signal; and the other to determine when to sample the wobble data. That is, in a CD-recordable system, after a timing loop (phase lock loop) is locked to the underlying wobble frequency and phase, it is still necessary to determine when to sample the resulting signal to obtain the wobble data. In addition, the Applicants have found that there is sometimes a frequency offset between the underlying wobble frequency and the wobble data due to manufacturing error or process deviation and the like, and the best time to sample the resulting signal would change with time. In this case there is an additional concern that the data should be placed on the disc with respect to the wobble data, rather than the wobble signal, in order to avoid an overwriting of the user data or a gap in the user data.
FIG. 2 schematically illustrates a conventional two-PLL system 20 for a CD-recordable format, including a first PLL 22 for a wobble signal, and a second PLL 24 for the wobble data. The output of the wobble signal PLL 22 reflects the underlying frequency modulation (wobble phase output), which is used as an input to the second PLL 24. For example, in the second PLL 24, the wobble phase output may be high-pass filtered to remove any DC content, and then squared to remove the effect of the sign of the signal. The output of the second PLL 24 will be used as a clock to sample the input signal, and from these samples, the signal can be demodulated. Thus, the arbitrary phase offset and any possible frequency offsets are eliminated by the second PLL 24.
However, compared with a DVD-recordable system, the conventional two-PLL CD-recordable system has disadvantages that it requires additional circuitry for the second PLL, which increases the size and cost of the device. In addition, the presence of multiple circuits with the same function increases the design complexity, as well as the pin count of the chip, the number of components required to support the chip, and the like.